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Question: Primary amines \({\text{(RN}}{{\text{H}}_{\text{2}}}{\text{)}}\) reacts with nitrous acid to give: ...

Primary amines (RNH2){\text{(RN}}{{\text{H}}_{\text{2}}}{\text{)}} reacts with nitrous acid to give:
A. RNH3+NO2{\text{RNH}}_3^ + {\text{N}}{{\text{O}}_{\text{2}}}
B.ROH{\text{ROH}}
C. ROR{\text{ROR}}
D.none of these

Explanation

Solution

We should know how nitrous acid and amine behaves. The nitrous acid gives NO + {\text{N}}{{\text{O}}^{\text{ + }}} cation. The nitrogen of amine works as a nucleophile. The primary amine can both hydrogen and nitrogen as nitrogen gas to give an alkyl cation. Alkyl can be treated with water.

Complete Answer :
The amines are basic due to the presence of lone pairs on nitrogen, so behave as a nucleophile. Nitric acid generates electrophile NO + {\text{N}}{{\text{O}}^{\text{ + }}}. The nucleophilic amine attacks on electrophile NO + {\text{N}}{{\text{O}}^{\text{ + }}} so, the nitrogen of amine gets positive change thus loses one proton. Then, the second proton from the nitrogen of amine transfers to the oxygen of electrophile NO + {\text{N}}{{\text{O}}^{\text{ + }}} forming a nitrogen-nitrogen double bond R − N = N − OH{\text{R}}\, - \,{\text{N = N}}\, - \,{\text{OH}}. Then –OH abstract protons and lose water molecules to form R − N2+{\text{R}}\, - \,{\text{N}}_2^ + . Nitrogen gas removed to give alkyl cation.

The product of the reaction of a primary amine (RNH2){\text{(RN}}{{\text{H}}_{\text{2}}}{\text{)}} with nitrous acid is shown as follows:
RNH2  + HNO2 → ROH  + N2 + H2O{\text{RN}}{{\text{H}}_{\text{2}}}\,{\text{ + HN}}{{\text{O}}_{\text{2}}}\, \to \,{\text{ROH}}\,\, + \,{{\text{N}}_2}\, + \,{{\text{H}}_{\text{2}}}{\text{O}}

The reaction of primary amine (RNH2){\text{(RN}}{{\text{H}}_{\text{2}}}{\text{)}} with nitrous acid gives primary alcohol, water and nitrogen gas. Nitrogen is colourless and odourless gas. By the evolution of the gas, primary amines are identified.
So, primary amines (RNH2){\text{(RN}}{{\text{H}}_{\text{2}}}{\text{)}} reacts with nitrous acid to give alcohol ROH{\text{ROH}}.

Therefore, option (B) ROH{\text{ROH}}, is correct.

Note: The different amines react differently with nitrous acid. Primary amines form alcohol. Secondary amines form nitroso compounds. The secondary amine gives N-alkyl-N-nitroso alkylamine and water. The N-alkyl-N-nitroso alkylamine is yellow coloured oil. So, secondary amines are identified by the formation of a yellow oil. The tertiary amine does not react. This reaction is used for distinguished primary, secondary and tertiary amine compounds.